For a long-lasting paint job, a clean wall is crucial. Ask how they plan to clean your walls, if at all, before getting started. A high-quality company will thoroughly prepare a wall so that the paint will lay smoothly and last a long time.

Can I ask for references?

Yes. A well-established company will have more experience and likely more references for you to reach out to than a new company that's just getting off the ground. Be sure to ask for references so you can speak to former customers about their experience working with the painter. When speaking to these people, ask about:

Whether they had issues with the crew

If the painters were messy or neat

How the paint has held up since the project was completed

Do I have a choice of what kind I paint I want a painter to use?

The quality of the paint will affect how good the walls look when the project is completed. If your contractor says they plan on using a paint you don't like, feel free to request an alternative.

How important is a trained painter?

If the contractor makes a point to train new hires on safety techniques as well as quality standards on the job, you're probably in good hands.

What are some risks with hiring a painter?

Unfortunately, there are plenty of cases where a painter has stolen items from a home or caused damage to the house or injury to the homeowner. Subcontractors may not be as skilled as employees, and the contractor you're hiring may not have much experience working with them, making them more of a risk.
Additionally, ask if the painters are employees or subcontractors, and what the screening or hiring process is like.